This invention relates to apparatus for measuring the distribution of magnetic flux trapped in a superconducting magnetic shielding container which is required in such fields as Josephson computers and measurement of biomagnetics to attain high magnetic shielding, apparatus for detecting the distribution of magnetic flux trapped in a ground plane of Josephson circuits, and apparatus for detecting presence and the position of small magnetic flux, that is, magnetic flux quantum trapped in a superconductor.
A scanning electron microsope has been used to detect position of the magnetic flux quantum trapped within a Josephson junction. In this method, position of trapped magnetic flux is determined from the correlation between focus of the electron beam and variation of the maximum current value of the Josephson junction. However, this method is available only to the Josephson junctions, thus it is not applicable to a simple plate of superconductor and a ground plane or magnetic shield.
Another method for detecting the trapped magnetic flux has been developed. In this method, a superconductor to be detected is used as an electrode and a row of counter-electrodes is disposed on the electrode with an insulator between. The trapped magnetic flux is detected by sensing variation of the maximum current. However, it needs a specially prepared sample.
When the surface of the superconductor is scanned with a pick-up coil (by shifting the superconductor or the pick-up coil), the magnetic field component, perpendicular to the surface of the superconductor is detected, which detects only the trapped magnetic flux because the outer magnetic field is parallel to the surface of the superconductor due to the Meissner effect. In this case, the larger the distance between the pick-up coil and the surface, the more difficult the detection of the trapped magnetic flux. Therefore, it is preferable to keep the distance about 10.about.100 .mu.m. But, it is impossible to previously set this distance at the room temperature because of the non-uniform thermal contraction. If the distance between the surface of the superconductor and the pick-up coil is varied during the scanning of the pick-up coil, the intensity of the detected signals is also varied to cause noises. Thus, means for maintaining the distance is indispensable. In the case that a spacer is provided between the pick-up coil and the surface of the superconductor for maintaining the distance, the vibration due to the sliding will cause noises. Further rubbing by the spacer might damage the the surface of the superconductor. This damage which makes the inhomogeneity of the thickness will cause the magnetic flux to be trapped. Therefore, the spacer is not adequate for maintaining the distance.
A main object of this invention is to provide means for resolving the above problems.